Not drinking enough may contribute to headaches in some people.
MedlinePlus includes headache among symptoms of mild to moderate dehydration.
The exact mechanism can vary, and headaches are extremely nonspecific. Poor sleep, stress, migraine, illness, caffeine withdrawal, and many other factors can also be responsible.
If drinking fluids does not improve a persistent or severe headache, another cause should be considered.
5. You May Become Dizzy or Lightheaded
Dehydration reduces the amount of fluid circulating in the body.
If blood volume drops enough, blood pressure can fall, especially when standing up quickly.
This may cause:
Dizziness
Lightheadedness
Feeling faint
Unsteadiness
Dizziness is a recognized sign of dehydration, and severe fluid loss can cause low blood pressure.
If dizziness is severe, recurrent, or accompanied by fainting, chest pain, or neurological symptoms, medical evaluation is important.
6. Your Heart May Beat Faster
